The present invention relates to isotope separation and enrichment and, more particularly, to a method wherein a molecular mixture containing at least 3 isotopes is vibrationally excited and induced to chemically react and provide a product in which the middle mass isotope is enriched.
It is well-known that very high energy vibrational quantum states of some molecular gases can be substantially populated under specific nonequilibrium conditions by the mechanism of vibration-vibration (V-V) exchange pumping. This kinetic mechanism is automatically operative in high pressure, collision-dominated gases wherever a sufficiently large amount of energy can be put into the vibrational mode of a gas molecule, while, simultaneously, a low gas kinetic (translational/rotational mode) temperature is maintained. The vibrational energy state population distribution in such cases is highly non-Boltzmann and characterized by relative overpopulation of the higher, anharmonic, vibrational quantum states with respect to the lowest vibrational states. This type of nonequilibrium vibrational population distribution is creaated by vibration-vibration (V-V) inelastic collisions between the gas molecules, in which quanta of vibrational energy are exchanged by the anharmonic vibrational modes of the collision partners. This process is the major inversion mechanism in electrically excited, infrared carbon monoxide lasers.
Methods for enriching selected isotopes based on vibration-vibration pumping are known in the art. In these methods, an isotopic mixture is subjected to excitation radiation which causes the heavier molecular weight isotopes to assume higher vibrational energy levels. Although the difference in molecular weight is small, there is a significant difference in the degree of vibrational excitation of the isotopes. As a result, the heavier isotopes preferentially dissociate or otherwise preferentially react to produce a product in which the heavier isotope is more concentrated.
U.S. Pat. No. 4,142,955 to Rich et al teaches a method for enriching isotopes where an isotopic mixture of gases is irradiated with a pump laser which produces radiation capable of resonant absorption by at least one species of the mixture. That species populates the higher vibrational states by vibration-to-vibration pumping. In one embodiment, the mixture of gases contains carbon monoxide which dissociates and forms C.sub.2 that is isotopically enriched in C.sup.13.
U.S. Pat. No. 4,092,405 to von Rosenberg, Jr. et al discloses a method for separating deuterium from hydrogen wherein hydrogen containing its natural abundance of deuterium is supplied to a reaction vessel with a reactant such as bromine. In the reaction vessel, the hydrogen and deuterium are vibrationally excited such that vibration-vibration energy pumping occurs from which follows chemical reactions that produce HBr and DBr at such relative rates that the mixture is enhanced in deuterium content. Vibrational excitation is accomplished through an ionization sustained, electrical discharge.
The aforementioned methods are effective in separating the heavier isotope or isotopes in an isotopic mixture. A method known as the "inverse isotope effect," described by Marcheret et al, "Isotope Effect in the Kinetics of Nonequilibrium Plasmochemical Reactions," Sov. Phys. Dokl. 25, 925 (1980) is useful in enrichment of lighter isotopes. In this method, the lighter isotope pumps to higher vibrational levels and predominantly enters into reaction. Thus, means for preferentially separating the heavy and light isotopes from a mixture via vibration-vibration pumping are reported in the literature.